Venus

Venus basic data Semi-major axis 0.72 AU Eccentricity 0.0068 Orbital tilt 3.39° Orbital Period 225 Earth days Rotation Period 243 Earth days (retrograde) Diameter 95% Earth’s Mass 82% Earth’s Density 5.2 g/cm3 Atmospheric Pressure 90 atm at surface Temperature 733 K (hottest planet) Temp is almost uniform due to thick atmosphere.

Strong upper level winds of up to 350 km/h Atmosphere so thick, can’t see surface, at visible, UV or most IR wavelengths Strong upper level winds of up to 350 km/h Convection: hot air from equatorial regions to poles, cools, returns to equator. Keeps T very constant. Convection and winds give V-shape appearance. Yet at surface winds are only < 5 km/h due to intense pressure. Visible UV IR

Missions to Venus Soviet Venera 4 -18 (1967 - 1983) Mariner 2, 5 and 10 (1962, 1967 and 1974) Pioneer Venus (1978) Magellan (1989) Venus Express (ESA, launched 9 Nov 2005), in orbit since May 2006. Ended in 2014. Venus Express studying atmosphere.

Venus' atmosphere Hot, dry, dense: so hot at surface (~733K) it almost melts rocks Thick atmosphere, 96.5% CO2 (strong greenhouse effect => high temps) 0.15% sulfuric acid (H2SO4), responsible for clouds and haze

So how can we study surface through the thick clouds? Venus’ clouds consist of sulfuric acid (H2SO4) Max height of Earth’s clouds At lower levels, SO2 droplets evaporate, so no rain. So how can we study surface through the thick clouds? Radar  long wavelength radio waves will pass through clouds.

Radar echo measures altitude space probe time for signal to return tells you the altitude of surface feature. Different substances also reflect radar to different degrees. Strength of return signal gives clue to surface composition. Planet Surface

Topographic map of Venus Flatter than Earth, no evidence of plate boundaries. => No large scale plate tectonics Earth: continents and oceans differ by 4-6 km. Venus: 60% of surface within 500m of avg. 85% plains 15% highland plateaus (Ishtar Terra and Aphrodite Terra)

The surface of Venus: Magellan radar images Mountains, volcanoes, lava flows and impact craters indicate active interior and a young surface

Impact Craters Only ~1000 seen, spread randomly (unlike the Moon): All parts have about the same age. None < 3 km across (no meteor impacts < 30 m), smaller ones burn up in atmosphere Paucity of impact craters compared to Moon, Mercury implies that the surface is young, roughly 500 million years.

Field of craters and the largest crater found on Venus (280 km diameter) Mead Crater Rougher surface provides brighter radar echo => recent, unweathered event

More recent lava flows also rougher

Volcanism Volcanoes also randomly distributed – no plate tectonics. SO2 varying in lower atmos at 40% level (Venus Express), but debated whether this means ongoing volcanism. SO2 should disappear in 10-20 Myr, so volcanism as recent as that. Lightning not accepted as indication of volcanism. Volcanoes also randomly distributed – no plate tectonics.

Shield volcano elevation map from Magellan radar data Shield volcano elevation map from Magellan radar data. About 100 km across. Vertical scale exaggerated by factor of 22.5 Sif Mons, and a ~ 5 km long lava flow. Estimate lava flowed within 10 million years.

"Pancake volcanoes" - eruptions of lava through vents close to ground. "Coronae" - Concentric pattern, with a radial fracture => lava from inside caused surface to stretch.

Yet another type, a tick volcano, with ridges and valleys Yet another type, a tick volcano, with ridges and valleys. A flat summit ~22 mi diameter.

More evidence for past (. ) volcanic activity: More evidence for past (?) volcanic activity: Venera 13 (Soviet lander 1981) found rocks similar to Earth’s basalt. Veneras 7-14 attempted to land. Didn’t last for long before heat and pressure got them. Venera 13 survived 2 hrs and sent photos

A 2 km-wide channel, caused by lava. Length is 6800 km (= 4200 miles) A 2 km-wide channel, caused by lava. Length is 6800 km (= 4200 miles). Compare to Nile at 6600 km. Carved by hot lava, which should remain liquid for a long time due to extreme surface temperatures

More evidence of small-scale surface deformation 1 km Cross-hatched structure. Faults and fractures? Details not known.

Is volcanism still ongoing? Venus Express infrared imaging in an atmospheric window at 1.02 microns reveals nine sites of hot-spot volcanism within past 2.5 million years and possibly 250,000 years S02 levels in atmosphere indicate recent volcanism, but may not constrain time to < 10 million years ago. High emissivity of basalt at IR apparently indicates young, unweathered flows. Gets dimmer as it ages.

Stronger, more recent (summer 2015) evidence from Venus Express: Some near-IR wavelengths probe surface as well as atmosphere. Dominated by blackbody radiation. IR spectrum yields temperature. Some changes in brightness found on a few surface features: Temperature is changing, suggesting volcanic activity. “Object A” reached 1100 K and is about 1 km across.

Volcanism summary No volcanoes in chains => no plate tectonics Much evidence for localized upwelling and fracturing Volcanoes may still be active Surface is young: roughly 500 million years. All parts have about same age. If volcanoes active, only small fraction of surface affected.

Why different from Earth? One idea: more active volcanism keeps crust thin. Rock may even be soft due to surface heat. Easy to break through (“flake tectonics”). Con: very few deformed craters

Second idea: the surface undergoes uniform upheaval about every ½ billion years. Heat builds up under thick, dead lithosphere until catastrophic surface meltdown. Seismometers would be very nice to have on Venus…

Interior presumably like Earth’s in terms of densities Interior presumably like Earth’s in terms of densities. Iron core, mantle, crust  No magnetic field

Climate History of Venus Like Earth, started with outgassing from volcanoes (mostly H2O, CO2, SO2) and perhaps additional water from cometary impacts. Young Sun only 70% as luminous as now, so Venus’ early atmosphere would have been cooler. Probably liquid water? At least some water would have been gaseous. CO2 dissolved in water and rocks. Water vapor is greenhouse gas, so T would rise. Plus aging Sun got brighter.

Runaway Greenhouse Effect! Hotter Sun + water vapor increases atmospheric temperature. Water could no longer be liquid as Sun got hotter (~ few 100 million years). Oceans start to evaporate, adding more H2O and CO2 (and SO2) to atmosphere. Stronger Greenhouse effect => further evaporation => stronger Greenhouse effect, etc. Eventually, CO2 even baked out of rocks into atmosphere. Temperature stops rising when all CO2 removed. Runaway Greenhouse Effect!

What happens to H2O? Solar UV radiation breaks H2O apart, H lost, O reacts to form molecules. Chain of reaction(s) to make sulfuric acid, skip

Comparison of Venus and Earth Both display volcanism and geological activity, BUT Venus has: Slow rotational period No plate tectonics No magnetic field No satellite No water High surface temperature and dense atmosphere Rocks above Curie T, no frozen in B fields to be detected Was Runaway Greenhouse Effect inevitable at this distance from Sun?

Venus fly-over

Venus Express Aim: to study atmosphere Interactions surface Interactions with solar wind Circulation and composition as a function of depth Radiative balance IR image of doubled-eyed vortex at south pole Will operate until May 2009 About 60 km

IR images, April 2006. 2.3 and 1.7m Atmospheric structure at 35 and 20 km altitude respectively Stripes: wave-like atmospheric motion (tidal forces?) but nature still unknown.

ASPERA (Analyzer of Space Plasma and Energetic Atoms), data taken when flying through interaction regions between Venus and solar wind. Left: heated solar wind (protons and alpha-particles) Right: regions of massive 'escape' of oxygen ions

Slow, retrograde motion

Retrograde rotation hard to understand just from planetary formation from pre-solar system nebula – almost everything spins and orbits in same sense Possible explanations: 1) Rather complex mechanism involving tidal interaction Venus, Sun & Earth, and atmospheric braking ? 2) Massive impact de-spinning Venus (no moons?)

Rift valleys Similar in size to East African Rift (largest on Earth, tectonic motion between African and Eurasian plates). On Venus, rift valleys consequence of local activity – no large scale plate tectonics.